A thin glacial diamicton, informally termed Granite drift, occupies the floor of central Beacon Valley in southern Victoria Land, Antarctica. This drift is Ͻ1.0 m thick and rests with sharp planar contacts on stagnant glacier ice reportedly of Miocene age, older than 8.1 Ma. The age of the ice is based on 40 Ar/ 39 Ar analyses of presumed in situ ash-fall deposits that occur within Granite drift. At odds with the great age of this ice are high-centered polygons that cut Granite drift. If polygon development has reworked and retransported ash-fall deposits, then they are untenable as chronostratigraphic markers and cannot be used to place a minimum age on the underlying glacier ice.Our results show that the surface of Granite drift is stable at polygon centers and that enclosed ash-fall deposits can be used to define the age of underlying glacier ice. In our model for patternedground development, active regions lie only above polygon troughs, where enhanced sublimation of underlying ice outlines high-centered polygons. The rate of sublimation is influenced by the development of porous gravel-and-cobble lag deposits that form above thermal-contraction cracks in the underlying ice. A negative feed-*back associated with the development of secondary-ice lenses at the base of polygon troughs prevents runaway ice loss. Secondaryice lenses contrast markedly with glacial ice by lying on a ␦D versus ␦ 18 O slope of 5 rather than a precipitation slope of 8 and by possessing a strongly negative deuterium excess. The latter indicates that secondary-ice lenses likely formed by melting, downward percolation, and subsequent refreezing of snow trapped preferentially in deep polygon troughs.The internal stratigraphy of Granite drift is related to the formation of surface polygons and surrounding troughs. The drift is composed of two facies: A nonweathered, matrix-supported diamicton that contains Ͼ25% striated clasts in the Ͼ16 mm fraction and a weathered, clast-supported diamicton with varnished and wind-faceted gravels and cobbles. The weathered facies is a coarsegrained lag of Granite drift that occurs at the base of polygon troughs and in lenses within the nonweathered facies. The concentration of cosmogenic 3 He in dolerite cobbles from two profiles through the nonweathered drift facies exhibits steadily decreasing values and shows the drift to have formed by sublimation of underlying ice. These profile patterns and the 3 He surface-exposure ages of 1.18 ؎ 0.08 Ma and 0.18 ؎ 0.01 Ma atop these profiles indicate that churning of clasts by cryoturbation has not occurred at these sites in at least the past 10 5 and 10 6 yr. drift is stable at polygon centers, low-frequency slump events occur at the margin of active polygons. Slumping, together with weathering of surface clasts, creates the large range of cosmogenic-nuclide surface-exposure ages observed for Granite drift. Maximum rates of sublimation near active thermal-contraction cracks, calculated by using the two 3 He depth profiles, range from 5 m/m.y. to 90 m/m.y. Sublimat...

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Recently, widespread valley-bottom damming for water power was identified as a primary control on valley sedimentation in the mid-Atlantic US during the late seventeenth to early twentieth century. The timing of damming coincided with that of accelerated upland erosion during post-European settlement land-use change. In this paper, we examine the impact of local drops in base level on incision into historic reservoir sediment as thousands of ageing dams breach. Analysis of lidar and field data indicates that historic milldam building led to local base-level rises of 2–5 m (typical milldam height) and reduced valley slopes by half. Subsequent base-level fall with dam breaching led to an approximate doubling in slope, a significant base-level forcing. Case studies in forested, rural as well as agricultural and urban areas demonstrate that a breached dam can lead to stream incision, bank erosion and increased loads of suspended sediment, even with no change in land use. After dam breaching, key predictors of stream bank erosion include number of years since dam breach, proximity to a dam and dam height. One implication of this work is that conceptual models linking channel condition and sediment yield exclusively with modern upland land use are incomplete for valleys impacted by milldams. With no equivalent in the Holocene or late Pleistocene sedimentary record, modern incised stream-channel forms in the mid-Atlantic region represent a transient response to both base-level forcing and major changes in land use beginning centuries ago. Similar channel forms might also exist in other locales where historic milling was prevalent.

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We surveyed adjacent reaches with differing riparian vegetation to explain why channels with forested banks are wider than channels with nonforested banks. Cross sections and geomorphic mapping demonstrate that erosion occurs at cutbanks in curving reaches, while deposition is localized on active fl oodplains on the insides of bends.Our data indicate that rates of deposition and lateral migration are both higher in nonforested reaches than in forested reaches. Two dimensionless parameters, α and E, explain our observations. α represents the infl uence of grassy vegetation on rates of active fl oodplain deposition; it is 5 times higher in nonforested reaches than in forested reaches. E is proportional to rates of cutbank migration; it is 3 times higher in nonforested reaches than in forested reaches. Differences in width between forested and nonforested reaches are proportional to E/α. In forested reaches, channels are wide with banks that are diffi cult to erode. Dense tree roots create a low value of E, and the channel migrates slowly. E/α is high, however, because α is very low: shade from trees inhibits the growth of grass on active fl oodplains. In nonforested reaches, channels are narrow with banks that are easy to erode. E is high, and the channel migrates rapidly. E/α is low, however, due to a very large value of α: grass grows readily on nonforested convex bank fl oodplains. Thus, differences in width between forested and nonforested reaches are related to a balance between rates of cutbank erosion and rates of deposition on active fl oodplains, implying that equilibrium widths develop to equalize rates of cutbank erosion and vegetationmediated rates of deposition on active fl oodplains. These results suggest that accurate models of width adjustment should consider the combined effects of bank erodibility and fl oodplain depositional processes, rather than focusing on these processes in isolation from one another.

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Rock glaciers, common in many alpine and polar regions, have poorly understood internal structure, dynamics, and origins. A renewal of interest in the climatic and geomorphic significance of these striking landforms has served to intensify a long-standing controversy surrounding the genesis of rock glaciers. The controversy, which began more than 30 years ago, has resolved into two primary viewpoints. One holds that rock glaciers form through a continuum of glacial to periglacial processes and encompass features that vary from debris-covered glaciers to slightly remobilized talus or till. The opposing view holds that all rock glaciers are exclusively features of creeping permafrost, genetically distinct from glaciers. Several factors have prolonged this debate: (1) sparse direct observations of internal composition and processes of ice formation;(2) few long-term measurements of rock glacier deformation; (3) difficulties in establishing geophysical, geochemical, or petrographic methods that unequivocally distinguish between ice of glacial and periglacial origins; (4) difficult access and remote locations of most rock glaciers; and (5) often arbitrary terminological distinctions between "glacial" and "periglacial" processes. Results from several recent studies, some presented in this volume, demonstrate conclusively that at least some rock glaciers are glacigenic, making untenable the view of rock glaciers as exclusively periglacial. This conclusion indicates that several previously held concepts of rock glacier dynamics and development should be re-evaluated. In addition, it highlights the need for researchers to move beyond taxonomic arguments, and to improve understanding of fundamental aspects of rock glaciers such as climatic sensitivity, geochemistry, hydrology, dynamics, structure, mass balance, and genetic and spatial variability.